1. Field of the Invention
This invention relates to the field of operational amplifiers (op amps) input stages, and particularly to common mode linearized input stages.
2. Description of the Related Art
The open loop gain of an op amp decreases with increasing input signal frequency. This requires that a larger error signal appear across the amplifier's input for signals of a given amplitude and increasing frequency. This, in turn, requires the op amp to have a large linear input range; if the input range is insufficient, distortion will result.
A bipolar differential transistor pair has a highly nonlinear input range. One way to extend the linear portion of a bipolar pair's input range is with the use of degeneration resistors. However, the use of degeneration resistors can result in noise degradation, as well as loss of headroom.
Another approach to extending linear input range requires the use of class AB-type input stages. Unfortunately, such stages are also known to increase noise and lower precision.
Yet another approach to extending linear input range is to use a series-connected doublet configuration which employs the “multi-tanh principle”, as described, for example, in Gilbert, IEEE Journal of Solid-State Circuits, Volume 33, Issue 1, January 1998, pp. 2–17. Here, individual nonlinear (hyperbolic tangent, or tanh) transconductance functions can be separated along the input-voltage axis to achieve a much more linear overall function. A typical implementation is shown in FIG. 1. However, though this arrangement extends the linear input range, it requires dividing resistors Ra and Rb between its inputs.
An op amp's linear input range can also be affected by manipulating its tail currents. One tail-current modulating approach is described in U.S. Pat. No. 5,214,329 to Furino, Jr. Here, the amplifier's bias currents are modulated as a function of the differential input voltage. However, Furino modulates bias currents through a direct measurement of differential input voltage, which can add current noise and offset current to the differential inputs.